Researchers started with data from surveys of adults in the United States that asked how much coffee they consumed, as well as other foods and drinks, and then they looked at their rates of death and disease over the following two decades.

The study was large, including more than 200,000 women and 50,000 men.

At first, researchers did not see an obvious relationship between coffee consumption and death rates. Study participants who drank between less than a cup of coffee and three cups a day had 5% to 9% lower risk of dying than those who drank no coffee. Those who drank more than three cups a day did not see any benefit. The finding was murky, like previous studies, some of which suggested a benefit and some did not.

But when the researchers looked at coffee consumption only among people who said they never smoked, the relationship became clearer: Those who drank between less than a cup of coffee and three cups a day had 6% to 8% lower risk of dying than noncoffee drinkers. Those who drank three to five cups and more than five cups had 15% and 12% lower death rates.

"The lower risk of mortality is consistent with our hypothesis that coffee consumption could be good for you (because) we have published papers showing that coffee consumption is associated with lower risk of type 2 diabetes and (heart) disease," said Ming Ding, a doctoral student in the Harvard School of Public Health department of nutrition. Ding is the lead author of the study, which was published on Monday in the journal Circulation.